1. Field of the Invention
The present invention relates to an obstacle (or intruder) detecting system and an obstacle (or intruder) detecting device for specifying an obstacle (or intruder) by emitting electrical waves from a leakage coaxial cable.
2. Description of the Related Art
In a conventional obstacle detecting device, leakage coaxial cables are laid on around an obstacle monitoring district (hereinafter referred to as “monitoring district”), and electrical waves are emitted from one of the leakage coaxial cables (hereinafter referred to as “first leakage coaxial cable”) to the other leakage coaxial cable (hereinafter referred to as “second leakage coaxial cable”) to detect an obstacle in the monitoring district.
A transmitter for generating a pulse-shaped signal (hereinafter referred to as “pulse signal”) is connected to one end of the first leakage coaxial cable, and a receiver which receives a signal obtained through delay of the pulse signal in accordance with the position on the second leakage coaxial cable is connected to one end of the second leakage coaxial cable. The one end of the second leakage coaxial cable is located at the same side as the one end of the first leakage coaxial cable.
Furthermore, a filter for picking up the envelope curve of the signal waveform from the pulse signal received by the receiver is connected to the receiver, and there are provided a storage device for pre-storing the envelope curve of the signal waveform when no obstacle exists, and a computing unit for detecting the position of an obstacle on the basis of the differential waveform between the envelope curve when no obstacle exists and the envelope curve picked up from the filter.
As described above, the position of an obstacle intruding into the monitoring district is detected by receiving the pulse signal from the transmitter, and the magnitude of the differential waveform between the envelope curves is dependent on the power of the detection signal which is received by the second leakage coaxial cable and passes from the receiver to the computing unit. Therefore, it is a challenge in enhancement of detection precision that the power of the detection signal is kept constant (fixed) on the time axis from the arrival of a detection signal from the transmitter connected to one end of the first leakage coaxial cable to generate a pulse signal at the receiver of the second leakage coaxial cable till the arrival of the detection signal at the computing unit.
Furthermore, when the first leakage coaxial cable laid on around the monitoring district and connected to the transmitter at one end thereof or the second leakage coaxial cable connected to the receiver at one end thereof falls into a characteristic state different from the normal characteristic state thereof due to some cause (for example, in the case of disconnection of the leakage coaxial cable or similar cases), the differential waveform between the envelope curve of the signal waveform picked up from the filter (hereinafter referred to as “detected envelope curve”) and the envelope curve of the signal waveform under the state that no obstacle exists (hereinafter referred to as “reference envelope curve”) becomes large, so that the present state is regarded as being identical to the state where an obstacle is detected (i.e., error detection).
Still furthermore, when the power of the detection signal received by the second leakage coaxial cable passes from the receiver to the computing unit is small, the differential waveform between the detected envelope curve and the reference envelope curve is small, so that the present state falls into an obstacle undetectable state (non-detection).    Prior Art Patent Document 1: JP-A-10-95338 (FIG. 1 and description thereof)
As described above, in order to enhance the detection precision in the conventional obstacle detecting device, it is required that the power of the detection signal from the time when it reaches the receiver from the transmitter for generating the pulse signal till the time when the detection signal reaches the computing unit from the receiver is made constant on the time axis. However, the constituent elements constituting the transmitter and the receiver have temperature-dependent characteristics and aged characteristics caused by continuous operation, and thus the conventional obstacle detecting device has a problem that the power is not constant, but varies on the time axis until it reaches the computing unit.
If the problem as described above exists, when the leakage coaxial cable has a characteristic different from the normal characteristic (stationary state) due to some cause, it cannot be identified which one of intrusion of an obstacle and characteristic variation of the leakage coaxial cable occurs. Accordingly, the obstacle detecting device is set to the same state as the state where an obstacle is detected, and thus error detection occurs.
Likewise, if the problem as described above exists, the differential waveforms when an obstacle exists and when no obstacle exists are not fixed. Accordingly, it is impossible to settle a threshold value for the differential waveform to determine detection of an obstacle, and thus there occurs a state that the differential waveform does not exceed the threshold value even when an obstacle actually intrudes (non-detection) or a state that the differential waveform exceeds the threshold value because an object moves at the outside of the obstacle monitoring district (this case is also referred to as “error detection”).
Likewise, if the problem as described above exists, when the output power of the transmitter (the power of the pulse signal) exceeds the permissible transmission power of the transmitter and when the reception power of the receiver (the power of the detected pulse signal) exceeds the permissible reception power of the receiver, the pulse signal (detection signal) transmitted from the transmitter and the detection signal which is received by the receiver and then reaches the computing unit become distorted signals. Therefore, when the detection signal reaches the computing unit, the computing unit cannot recognize it as the detection signal output from itself. Accordingly, it is impossible to form the envelope curve, and the present state falls into the state that the differential waveform does not exceed the threshold value (non-detection) even when an obstacle intrudes.
That is, the conventional device is required to be placed under such an environment that the temperature is stable at all time, and when it is used in the open air under which the temperature rapidly varies, error detection or non-detection may occur. Furthermore, error detection or non-detection may also occur due to aged deterioration, and thus it is required to perform maintenance at a short period, that is, correct the threshold value or the like. Furthermore, the velocity of the detection signal in the leakage coaxial cable is dependent on the specific dielectric constant in the leakage coaxial cable. Therefore, when the specific dielectric constant varies due to aged deterioration or the like, the velocity of the detection signal, varies. In connection with this variation, an aberration occurs in the conversion of the detection signal to the detection position, and thus there is a problem that the intrusion position cannot be accurately measured.